Vulcanized elastomers such as thermoset polyurethanes having a Shore A hardness in the 55 to 70 range have a disadvantage in that during their manufacture, a vulcanizing process must be performed which results in waste of substantial amounts of material. Furthermore, many application areas, for example, work boot, linings or tubes, require a rubber-like grip and/or extreme flexibility. Polyurethane elastomers with thermoplastic processability are especially useful in these fields.
Hardness levels of conventional polyurethane polymers may be adjusted to less than 70 Shore A by adding plasticizers. For example, commercially available 60 Shore A type polyurethane polymer may contain approximately 20% by weight dimethylglycol phthalate. In spite of good processability, a considerable disadvantage of the these plasticized polymers, e.g., with oils, lies in the fact that the plasticizer is extractable, that is the long-term stability of such products is very poor because the plasticizer may be lost by solvent extraction. Plasticizer volatization is also a problem especially when a plasticizer is stressed for a long period of time at high temperatures. Another technological disadvantage of plasticized polyurethanes lies in the fact that mixtures of polyurethane and plasticizer are often incompatible; in time, this also leads to exudation and volatilization of plasticizer over time from the polyurethane which results in considerable changes in the physical properties of the product.
An alternative for producing soft thermoplastic polyurethane lies in substantially reducing the polyisocyanate:polyol ratio, for example, to obtain a 60 Shore A product, this ratio should be fixed at less than about 30:100. Such a small proportion of isocyanate, however, may substantially reduce the tensile strength of the product. In addition, the fluidity of the product may be decreased to the extent that processibility is adversely affected, particularly in injection molding processes where decreased fluidity results in long cycle times and an undesirable sticking of the product to molds and other equipment. Moreover, such products often undergo a substantial shrinkage which, of course, causes problems in tool production and design.